The present invention relates to process devices. More specifically, the present invention relates to field-mounted process control and measurement devices.
Process devices are used to measure and control industrial processes such as the refining of petrochemicals, the processing of food, the generation of electric power, and a number of other processes. Process measurement devices include process variable transmitters, which measure a process variable such as pressure or temperature and communicate the measured variable to a process controller. Another type of process device is an actuator, such as a valve controller or the like. Generally, process control is accomplished using a combination of transmitters, actuators, and a process controller that communicate across a process control loop to a controller. Both types of process devices interact with the physical process through process interface elements. Process interface elements are devices which relate electrical signals to physical process conditions, and include devices such as sensors, limit switches, valve controllers, heaters, motor controllers, and a number of other devices.
The process controller is typically a microcomputer located in a control room away from the process. The process controller can receive process information from one or more process measurement devices and apply a suitable control signal to one or more process control devices to influence the process and thereby control it.
In order to couple to the process, transmitters and actuators are generally mounted near the process in the field. Such physical proximity can subject the process devices to an array of environmental challenges. For example, process devices are often subjected to temperature extremes, vibration, corrosive and/or flammable environments, and electrical noise. In order to withstand such conditions, process devices are designed specifically for “field-mounting.” Such field-mounted devices utilize robust enclosures, which can be designed to be explosion-proof. Further, field-mounted process devices can also be designed with circuitry that is said to be “intrinsically safe”, which means that even under fault conditions, the circuitry will generally not contain enough electrical energy to generate a spark or a surface temperature that can cause an explosion in the presence of an hazardous atmosphere. Further still, electrical isolation techniques are usually employed to reduce the effects of electrical noise. These are just a few examples of design considerations, which distinguish field-mounted process devices from other devices, which measure sensor characteristics and provide data indicative of such characteristics.
Aside from the environmental considerations listed above, another challenge for field-mounted devices is that of wiring. Since process devices are located near the process far from the control room, long wire runs are often required to couple such devices to the control room. These long runs are costly to install and difficult to maintain.
One way to reduce the requisite wiring is by using two-wire process devices. These devices couple to the control room using a two-wire process control loop. Two-wire devices receive power from the process control loop, and communicate over the process control loop in a manner that is generally unaffected by the provision of power to the process device. Techniques for communicating over two-wires include 4-20 mA signaling, the Highway Addressable Remote Transducer (HARTS) Protocol, FOUNDATION™. Fieldbus, Profibus-PA and others. Although two-wire process control systems provide wiring simplification, such systems provide a limited amount of electrical power to connected devices. For example, a device that communicates in accordance with 4-20 mA signaling must draw no more than 4 mA otherwise the devices current consumption would affect the process variable. The frugal power budget of two-wire process devices has traditionally limited the functionality that could be provided.
Another way that the process control industry has reduced field wiring and reduced the number of transmitters is by providing transmitters with multiple sensor inputs. Such transmitters reduce the number of transmitters/sensor and thereby reduce wiring costs as well as overall system costs.
One technique is to employ multiple sensors with a single transmitter. These sensors can be coupled to measurement circuitry using a multiplexer. One such transmitter is the 848T available from Rosemount Inc.
One problem associated with the use of a multiplexer to couple measurement circuitry to different sensors occurs as a result of potential differences between the sensors. Such voltage potentials may introduce errors in the measurement and damage circuitry of the transmitter.